libexpr: remove matchAttrs boolean from ExprLambda

The boolean is only used to determine if the formals are set to a
non-null pointer in all our cases. We can get rid of that allocation and
instead just compare the pointer value with NULL. Saving up to
sizeof(bool) + platform specific alignment per ExprLambda instace.
Probably not a lot of memory but perhaps a few kilobyte with nixpkgs?

This also gets rid of a potential issue with dereferencing formals based on
the value of the boolean that didn't have to be aligned with the formals
pointer but was in all our cases.

src/libexpr/eval.cc

@@ -1316,13 +1316,13 @@ void EvalState::callFunction(Value & fun, Value & arg, Value & v, const Pos & po
 
  auto size =
  (lambda.arg.empty() ? 0 : 1) +
- (lambda.matchAttrs ? lambda.formals->formals.size() : 0);
+ (lambda.hasFormals() ? lambda.formals->formals.size() : 0);
  Env & env2(allocEnv(size));
  env2.up = fun.lambda.env;
 
  size_t displ = 0;
 
- if (!lambda.matchAttrs)
+ if (!lambda.hasFormals())
  env2.values[displ++] = &arg;
 
  else {
@@ -1402,7 +1402,7 @@ void EvalState::autoCallFunction(Bindings & args, Value & fun, Value & res)
  }
  }
 
- if (!fun.isLambda() || !fun.lambda.fun->matchAttrs) {
+ if (!fun.isLambda() || !fun.lambda.fun->hasFormals()) {
  res = fun;
  return;
  }

src/libexpr/flake/flake.cc

@@ -231,7 +231,7 @@ static Flake getFlake(
  if (auto outputs = vInfo.attrs->get(sOutputs)) {
  expectType(state, nFunction, *outputs->value, *outputs->pos);
 
- if (outputs->value->isLambda() && outputs->value->lambda.fun->matchAttrs) {
+ if (outputs->value->isLambda() && outputs->value->lambda.fun->hasFormals()) {
  for (auto & formal : outputs->value->lambda.fun->formals->formals) {
  if (formal.name != state.sSelf)
  flake.inputs.emplace(formal.name, FlakeInput {

src/libexpr/nixexpr.cc

@@ -124,7 +124,7 @@ void ExprList::show(std::ostream & str) const
 void ExprLambda::show(std::ostream & str) const
 {
  str << "(";
- if (matchAttrs) {
+ if (hasFormals()) {
  str << "{ ";
  bool first = true;
  for (auto & i : formals->formals) {
@@ -348,7 +348,7 @@ void ExprLambda::bindVars(const StaticEnv & env)
 
  if (!arg.empty()) newEnv.vars[arg] = displ++;
 
- if (matchAttrs) {
+ if (hasFormals()) {
  for (auto & i : formals->formals)
  newEnv.vars[i.name] = displ++;
 

src/libexpr/nixexpr.hh

@@ -233,11 +233,10 @@ struct ExprLambda : Expr
  Pos pos;
  Symbol name;
  Symbol arg;
- bool matchAttrs;
  Formals * formals;
  Expr * body;
- ExprLambda(const Pos & pos, const Symbol & arg, bool matchAttrs, Formals * formals, Expr * body)
- : pos(pos), arg(arg), matchAttrs(matchAttrs), formals(formals), body(body)
+ ExprLambda(const Pos & pos, const Symbol & arg, Formals * formals, Expr * body)
+ : pos(pos), arg(arg), formals(formals), body(body)
  {
  if (!arg.empty() && formals && formals->argNames.find(arg) != formals->argNames.end())
  throw ParseError({
@@ -247,6 +246,7 @@ struct ExprLambda : Expr
  };
  void setName(Symbol & name);
  string showNamePos() const;
+ inline bool hasFormals() const { return formals != nullptr; }
  COMMON_METHODS
 };
 

src/libexpr/parser.y

@@ -324,13 +324,13 @@ expr: expr_function;
 
 expr_function
  : ID ':' expr_function
- { $$ = new ExprLambda(CUR_POS, data->symbols.create($1), false, 0, $3); }
+ { $$ = new ExprLambda(CUR_POS, data->symbols.create($1), 0, $3); }
  | '{' formals '}' ':' expr_function
- { $$ = new ExprLambda(CUR_POS, data->symbols.create(""), true, $2, $5); }
+ { $$ = new ExprLambda(CUR_POS, data->symbols.create(""), $2, $5); }
  | '{' formals '}' '@' ID ':' expr_function
- { $$ = new ExprLambda(CUR_POS, data->symbols.create($5), true, $2, $7); }
+ { $$ = new ExprLambda(CUR_POS, data->symbols.create($5), $2, $7); }
  | ID '@' '{' formals '}' ':' expr_function
- { $$ = new ExprLambda(CUR_POS, data->symbols.create($1), true, $4, $7); }
+ { $$ = new ExprLambda(CUR_POS, data->symbols.create($1), $4, $7); }
  | ASSERT expr ';' expr_function
  { $$ = new ExprAssert(CUR_POS, $2, $4); }
  | WITH expr ';' expr_function

src/libexpr/primops.cc

@@ -2365,7 +2365,7 @@ static void prim_functionArgs(EvalState & state, const Pos & pos, Value * * args
  .errPos = pos
  });
 
- if (!args[0]->lambda.fun->matchAttrs) {
+ if (!args[0]->lambda.fun->hasFormals()) {
  state.mkAttrs(v, 0);
  return;
  }

src/libexpr/value-to-xml.cc

@@ -135,7 +135,7 @@ static void printValueAsXML(EvalState & state, bool strict, bool location,
  if (location) posToXML(xmlAttrs, v.lambda.fun->pos);
  XMLOpenElement _(doc, "function", xmlAttrs);
 
- if (v.lambda.fun->matchAttrs) {
+ if (v.lambda.fun->hasFormals()) {
  XMLAttrs attrs;
  if (!v.lambda.fun->arg.empty()) attrs["name"] = v.lambda.fun->arg;
  if (v.lambda.fun->formals->ellipsis) attrs["ellipsis"] = "1";

src/nix/flake.cc

@@ -346,10 +346,10 @@ struct CmdFlakeCheck : FlakeCommand
  auto checkOverlay = [&](const std::string & attrPath, Value & v, const Pos & pos) {
  try {
  state->forceValue(v, pos);
- if (!v.isLambda() || v.lambda.fun->matchAttrs || std::string(v.lambda.fun->arg) != "final")
+ if (!v.isLambda() || v.lambda.fun->hasFormals() || std::string(v.lambda.fun->arg) != "final")
  throw Error("overlay does not take an argument named 'final'");
  auto body = dynamic_cast<ExprLambda *>(v.lambda.fun->body);
- if (!body || body->matchAttrs || std::string(body->arg) != "prev")
+ if (!body || body->hasFormals() || std::string(body->arg) != "prev")
  throw Error("overlay does not take an argument named 'prev'");
  // FIXME: if we have a 'nixpkgs' input, use it to
  // evaluate the overlay.
@@ -363,7 +363,7 @@ struct CmdFlakeCheck : FlakeCommand
  try {
  state->forceValue(v, pos);
  if (v.isLambda()) {
- if (!v.lambda.fun->matchAttrs || !v.lambda.fun->formals->ellipsis)
+ if (!v.lambda.fun->hasFormals() || !v.lambda.fun->formals->ellipsis)
  throw Error("module must match an open attribute set ('{ config, ... }')");
  } else if (v.type() == nAttrs) {
  for (auto & attr : *v.attrs)